Band saws mostly consist of two wheels arranged one above the other, sometimes also one besides the other, with a horizontal axis of rotation, over which a band saw blade is guided. One of the wheels is driven and, hence, moves the band saw blade in a longitudinal direction. Band saw blades are, for example, 10 m long and run with a velocity of about 30 to 45 m/s.
For making sure that the band saw blade runs stably, even if a sawing material, for example wood, is guided with a certain force with its front against a narrow, toothed side of the band saw blade, the band saw blade is mechanically held under tension with high forces. This is done by increasing the distance between the wheel axes after having applied the band saw blade upon the wheels.
Moreover, it is well known to push the band saw blade, which otherwise would run along a common tangent line interconnecting the wheels, in an outward direction by means of two mechanical guide elements arranged at a distance along the respective strand, such that the band saw blade extends parallel to the tangent line over a certain section. This results in that the free length of the band saw blade is reduced to the distance between the two guide elements, and that the band saw blade reacts with a higher resistance to a force acting laterally on it.
Band saw blades are exposed to different mechanical loads not only by such a bias but also by the sawing itself. These loads cause the band saw blade to evade. Depending on how the forces acting chaotically and irregularly on the band saw blade during sawing engage same, various evasion movements occur.
One first such evasion movement is directed opposite the feed movement. This evasion movement is conventionally countered by guiding the band saw blade over wheels being configured crowned at their periphery. The evasion movement is quite critical in view of the dimensional accuracy of the sawing operation and the quality of the surface generated during the sawing.
A second such evasion movement is directed laterally. This evasion movement is significantly more critical because it influences both the dimensional accuracy and the surface quality. In conventional band saws, this evasion movement, as already mentioned, is countered essentially only by a high tension of the band saw blade and by shortening the free length thereof.
Finally, it may happen that the band saw blade is twisted around its longitudinal axis.
All these evasive movements are disadvantageous in operation. On the one hand, they result in a stretching of the band saw blade and, on the other hand result in an increased wear. Furthermore, also the quality, i.e. the dimensional accuracy and the quality of the generated surface, i.e. the saw cut, are negatively affected when the band saw blade evades laterally during sawing or twists.
In order to keep such movements and deformations as small as possible, one has suggested various mechanical guides for the band saw blade. These guides are mostly configured as slide guides or as roller guides. These guides, however, have the disadvantage that they likewise cause wear due to friction.
For this reason one has already suggested a magnetic guide for a band saw blade.
Printed citation DE 201 05 845 U1 describes such a magnetic band saw positioning apparatus. This prior art apparatus essentially consists of a U-shaped guide, the legs of which extending on both sides of the band saw blade to be positioned. The guide as a whole is supported via springs against a machine-mounted bearing in the feed direction of the sawing material.
Two parallel rows each of opposing magnets, apparently permanent magnets, are integrated into the two legs of the guide, wherein the rows extend parallel to the longitudinal direction of the band saw blade. The one row is positioned besides the tooth base of the band saw blade teeth and the other row is positioned besides the rear edges of saw blade holes extending in a longitudinal direction. Nothing is said in the printed citation neither about the polarisation of the magnets nor their interaction with the band saw blade.
The legs of the guide are dimensioned so long and the guide is positioned relative to the band saw blade such that the rear side of the band saw blade keeps a distance to the flange interconnecting the legs. Thereby, with large feed forces, the band saw blade can be somewhat displaced in the feed direction against the action of the magnets, wherein also the resilient support finally has a limiting function.
The guide effects exclusively a support of the band saw blade opposite the feed direction, and, hence, only counteracts the not so critical evasive movements of the band saw blade in the feed direction. Lateral evasive movements and a twisting of the band saw blade are not prevented by the prior art guide which, therefore, does not contribute to the improvement of the saw cut quality with regard to dimensional accuracy and surface quality.
Printed citation SE 436 849 B describes a circular or band saw in which a force is exerted on the saw blade by means of two electromagnets positioned on opposite sides of the saw blade. By means of a sensor the lateral position of the saw blade is detected, is compared with a desired position, and the saw blade, as the case may be, is redirected into the desired position by corresponding excitation of the magnets. By doing so, a fluttering of the saw blade is prevented.